wifi: rtw89: correct aSIFSTime for 6GHz band

aSIFSTime is 10us for 2GHz band and 16us for 5GHz and 6GHz bands.
Originally, it doesn't consider 6GHz band and use wrong value, so correct
it accordingly.

Cc: stable@vger.kernel.org
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://msgid.link/20240430020515.8399-1-pkshih@realtek.com